Conventional high pressure decorative laminates are produced by heat and pressure consolidating a laminate assembly comprising, in superimposed relationship, a plurality (e.g. 3-9) of phenol/formaldehyde resin impregnated saturating kraft paper sheets, a melamine/formaldehyde resin impregnated decor sheet and, optionally, a melamine/formaldehyde resin impregnated overlay sheet in a multi-opening press at from about 120.degree.-250.degree. C. and 6.9 MPa-11.0 MPa. The laminate assemblies are pressed, in back-to-back relationship, with the aid of release sheets and press plates, such that from about 8-35 laminates can be produced from each press opening during each press cycle.
These conventional laminates are employed, after being sanded and cut to size, as decorative surfaces for desktops, tabletops, wall panels and the like by adhering them to a substrate such as particleboard with conventional adhesives such as contact cement or rigid setting urea adhesives. The thickness of these laminates runs from about 0.5-10 mm or more and they may be formulated in such a manner so as to render them post-formable by the application of heat.
While the conventional high pressure laminates have undoubtedly achieved a phenominal success over the past 40 years, the need still exists for laminates having the properties of these conventional laminates but of thinner, more flexible, less costly nature and producible utilizing lower pressures during lamination.
Prior art procedures such as those taught in U.S. Pat. Nos. 3,220,916; 3,218,225; and 3,589,974 disclose the use of standard phenolic resins only to impregnate the kraft core sheets in the production of high pressure decorative laminates. No mention is made therein of the use of phenolic resin impregnated kraft sheets at low pressure i.e. below 5.5 MPa nor is there any disclosure of the use of an acrylic resin in conjunction with the phenolic resin to obtain laminates that can be prepared by low pressure consolidation i.e. under 5.5 MPa with properties comparable to laminates approximately twice the thickness and consolidated at pressure in excess of 6.9 MPa.
U.S. Pat. No. 3,798,117 sets forth an unsuccessful attempt to treat standard saturating kraft with an acrylic resin and produce satisfactory laminates therefrom while U.S. Pat. Nos. 3,983,907 and 3,975,572 disclose the use of wet strength paper impregnated with a mixture of melamine/formaldehyde and acrylic resins but fail to disclose the presence of any phenolic resin, even though high pressure is employed to consolidate the laminate.
It can therefore be seen that the prior art fails to teach the impregnation of standard kraft paper with an acrylic modified phenolic resin, optionally, containing a melamine/formaldehyde resin, nor does it teach the manufacture of low pressure laminate therefrom.
With regard to the decor sheet of the instant laminates, U.S. Pat. Nos. 3,220,916; 3,218,225; 3,798,117; 3,545,769; 3,983,907 and 3,975,572 teach the use of melamine/formaldehyde resin-acrylic resin mixtures to impregnate decor sheets preparatory to the formation of laminates therefrom. The impregnated decor sheets, after being oven dried, are further treated by passing them through a fully alkoxymethylated melamine mix to provide a top coat.
It has been found, however, that melamine/formaldehyde resins, when combined with acrylic emulsions and used to treat decor sheets to resin levels high enough to provide surface protection in the resultant laminate, result in hazy surfaces. Even at resin levels as low as 35%, there exists a tendency to whiten or haze the laminate surface.
The prior art therefore fails to teach a two-resin system, as employed in producing the laminates of this invention, so as to obtain a clear surface appearance and flexibility at low pressure while avoiding the use of alkoxymethylated melamine resins.